1. Field of the Invention
The invention relates to an emission control device provided in an exhaust system of an internal combustion engine. More specifically, the invention relates to a pressure detection mechanism for an exhaust emission control device that is provided in the exhaust system for purifying a toxic component in exhaust.
2. Description of the Related Art
In a diesel engine or a lean burn gasoline engine, an operation range that performs engine operation that supplies an air-fuel mixture with a high air-fuel ratio (a lean atmosphere) for combustion makes up a major portion of the entire operation range. An exhaust system of this type of engine (an internal combustion engine) is generally provided with an absorbent (a catalyst) for absorbing nitrogen oxides (NOx) in the presence of oxygen.
Furthermore, a great deal of technology has been developed relating to the removal of particulates (also referred to as PM, or particulate matter) such as carbon that are emitted from diesel engines. One example is an exhaust particulate processing device provided with a filter-shaped exhaust trap equipped with a catalyst, which is interposed in an exhaust passage. This catalyst-equipped trap has a catalytic action that causes deposited exhaust particulates to self-combust. The Japanese Patent Laid Open Publications listed below disclose technology related to these catalyst-equipped traps.
Since self-combustion is not sufficient to allow regeneration of the trap, an exhaust particulate processing device for an internal combustion engine such as described in Japanese Patent Laid-Open Publication No. Hei 2-271022 uses a heater to perform automatic regeneration of the trap. This exhaust particulate processing device for an internal combustion engine is provided with a catalyst-equipped trap interposed in an exhaust passage of the internal combustion engine, an exhaust bypass passage shaped so as to bypass the catalyst-equipped trap, a bypass valve that opens and closes the exhaust bypass passage, a heater provided upstream of the catalyst-equipped trap, a mechanism for detecting a particulate deposition state of the catalyst-equipped trap using a pressure loss therein, a control mechanism for promoting combustion of deposited exhaust particulate by application of heat from the heater when a predetermined operating condition is reached, and a warning mechanism that operates when an excess deposition state is detected.
The warning mechanism of the exhaust particulate processing device for an internal combustion engine operates on detection of an excess deposition state and notifies a vehicle operator of an abnormality, so as to prompt a manual operation to regenerate the catalyst-equipped trap. Therefore, if for some reason an excess amount of exhaust particulate becomes deposited, the warning mechanism prevents melting loss of the catalyst-equipped trap or sudden stop of the internal combustion engine due to the vehicle operator continuing to operate the vehicle unaware that excess deposition has occurred.
Japanese Patent Laid-Open Publication No. Hei 4-128509 discloses an exhaust system that uses a reliable method to substantially reduce particulate discharge from an internal combustion engine. This exhaust system is provided with a main flow path and a bypass flow path for directing exhaust gas from an entrance portion of the system to an exit portion, a valve for directing the exhaust gas through one of the flow paths in a selective fashion, a filter for filtering the exhaust gas that has been directed through the main flow path, a regeneration portion member provided between the valve and the filter, an oxidization portion member provided downstream of the main flow path filter, and a control portion member, which causes the regeneration portion member to operate immediately after a predetermined state is detected, and which controls the flow of the exhaust gas so as to stop the regeneration portion member when a regeneration process is complete.
This exhaust system is provided with a sensor adjoining the filter in the main flow path for detecting a predetermined state. When a state is detected where the filter is sufficiently filled with particulate, the valve is switched so that the exhaust gas immediately passes through the bypass flow path, and a high-temperature diesel fuel burner is ignited to carry out the regeneration process. When the high-temperature diesel fuel burner is ignited, heated gas containing 11 to 15% oxygen is passed through the trap from the burner. The particulate collected in the trap is combusted and then passed through an oxidization catalyst to carry out further burning of incombustible carbohydrates, thereby enabling discharge into the atmosphere.
However, the exhaust particulate processing device for an internal combustion engine disclosed in Japanese Patent Laid-Open Publication No. Hei 2-271022 is provided with a first pressure sensor upstream of the trap and a second pressure sensor downstream of the trap. A particulate deposition state is detected from a pressure loss of the trap based on a pressure difference of these two sensors. The exhaust system disclosed in Japanese Patent Laid-Open Publication No. Hei 4-128509 is provided with a pressure difference sensor in the main flow path for detecting a pressure difference applied to the trap. An index that shows the degree of particulate mass filling the trap is calculated based on this pressure difference.
Each of the devices in the aforementioned Japanese Patent Laid-Open Publications is provided with a pressure sensor pipe upstream of the catalyst-equipped trap. Fuel is added upstream of the catalyst traps through a fuel addition nozzle, temporarily reducing the oxygen concentration in the exhaust and temporarily increasing the quantity of a reducing component (HC or the like) in the catalyst-equipped traps. When this kind of process is performed, a fuel component is mixed with the exhaust, making the pressure sensor pipe provided upstream of the catalyst-equipped trap susceptible to blockage by an exhaust deposit. Blockage of the pressure sensor pipe by an exhaust deposit makes detection of a correct pressure difference and, therefore, detection of an accurate PM deposit amount difficult.